LearnTronics
Power-Supply Ripple and Filter Capacitor
Recipe #31 — power-supply ripple and filter capacitor
The rectifier and filter capacitor used earlier deserve a closer look. A reservoir capacitor does not create perfectly flat DC; it repeatedly charges near waveform peaks and discharges into the load between them.
What ripple means
The remaining up-and-down variation on the DC output is called ripple voltage. More capacitance, less load current, or a higher ripple frequency reduces the amount of droop between charging peaks.
Full-wave rectifier reminder
With 60 Hz mains feeding a full-wave bridge, charging peaks occur about 120 times per second. A 1000 µF capacitor supplying 0.1 A would have an idealized ripple of roughly 0.83 V peak-to-peak before regulator effects and other losses are considered.
The capacitor current can be surprisingly large
The reservoir capacitor is recharged in short pulses near the waveform peaks. Real designs must consider capacitor ripple-current rating, diode surge current, transformer impedance, ESR, heat, and regulator headroom.